Such ventilation devices may comprise a ventilation valve arranged in the fuel tank for aeration and ventilation purposes. Within the housing of the ventilation valve, an aeration and ventilation opening has been placed, which for simplification purposes will be subsequently named ventilation opening. The ventilation valve is located in an upper tank wall, and in this case the ventilation opening is connected to the ambient air. This ventilation valve has been designed as a float valve—in other words, a float that carries a sealing element and works together with the ventilation opening located in the float valve's housing that has been placed so it can move from a closed position to a resting position. In its resting position, the float is arranged in a lower position of the housing and the sealing element is located away from the ventilation opening.
Such valves are generally made for closing ventilation openings so fuel will not spill out, but they have the purpose of limiting the filling level of the fuel tank. During the filling up process (which takes place through a filling pipe found in the fuel tank), the float is lifted by the lifting forces exerted by the rising fuel. When the float finally reaches its closing position, it presses the sealing element with more or less substantial force against the ventilation opening. From that moment on, the gas displaced by the fuel flowing into the tank—a fuel-air mixture—can no longer escape the tank through the ventilation valve or the ventilation opening. As a result of the fuel flowing into the tank, the fuel level keeps rising at first, which leads to the overfilling of the tank. In other words, the fuel volume displaced by the float increases, so the float now dips into the fuel more than is necessary, which causes the corresponding increase of the force being exerted on the sealing element. Finally, the fuel that flows through the filling pipe and into the tank rises and as soon as the fuel reaches a certain level, the shut-off level, the fuel causes the pump nozzle to shut off.
Due to the overfilling that goes on during the filling up process and sometimes also due to the slowly increasing pressure within the tank when the vehicle is running, the re-opening of the ventilation opening is coupled with hysteresis (re-filling hysteresis). This means that the ventilation opening will remain closed until the fuel reaches a lower level through consumption, the so-called opening level. Depending on the horizontal cross-sectional area of the fuel tank, this level will be reached only when several liters of fuel (i.e. at least 6) have been consumed. This is not only disadvantageous because there is a potentially damaging inner pressure in the tank for a relatively long running period, but also because in the car rental business this effect makes it impossible to re-fill the car after short trips and gasoline consumption must then be paid either by the next customer or the car rental company.
With this assumption in mind, the task of the present disclosure is to suggest a ventilation device for the fuel tank of a vehicle that will be able to overcome the disadvantages described above.